Control means for bidirectional rotary fluid motor system

ABSTRACT

A control means for a bidirectional, rotary fluid motor means constructed and arranged to have porting and fluid working chambers to provide rotation of a rotor in one direction or the opposite direction. The control means comprises a control valve connected to communicate with the fluid motor means and a pressurized fluid supply means and a fluid exhaust or return means so that pressurized fluid fills or &#39;&#39;&#39;&#39;floods&#39;&#39;&#39;&#39; the fluid motor, including the ports and working chambers, in the neutral or null position of the control valve to thereby cause equal and opposite torque forces on the rotor and prevent rotation of the latter, and in another operative position of the valve effect an imbalance of torque forces on the rotor so as to cause rotation of the rotor in one direction or the other with a minimum time lapse.

United States Patent [11] 3,614,275

[72] Inventor Richard S. Eibsen 2,725,918 12/1955 Deshler .1 91/59 Hillsdale, NJ. 3,016,021 1/1962 Rineer 4. 418/221 [21] App]. No. 876,247 3,175,469 3/1965 Kee 418/149 [22] Filed NOV. 13, 1969 3,298,284 1/1967 Alexander. 418/270 [45] Patented Oct. 19, 1971 3,426,648 2/1969 Fehlings 418/100 Assignee Cumsswrigm Corpomfion Primary Examiner-Martin P Schwadron Assistant Examiner-Irwin C. Cohen 54 CONTROL MEANS FOR BIDIRECTIONAL fif f ff *i ROTARY FLUID MOTOR SYSTEM m 7 Clams 6 Drawing Figs. ABSTRACT: A control means for a bidirectional, rotary fluid s2 U.S.C1. 418/206, means constructed and arranged to have porting and 413/210, 418/270, 417/315 fluid working chambers to provide rotation of a rotor in one Int. direction or the opposite direction control means com- 1 F016 21/ 12 prises a control valve connected to communicate with the 50 Field of Search 418/191, fl id means and a pressurized fluid supply means and a 205, 268, 270, 209, 210; 91/418- fluid exhaust or return means so that pressurized fluid fills or 466; 417/315 "floods" the fluid motor, including the ports and working chambers, in the neutral or null position of the control valve to [56] References cued thereby cause equal and opposite torque forces on the rotor UNITED STATES PATENTS and prevent rotation of the latter, and in another operative 744,050 11/1903 Comstock 418/205 position of the valve effect an imbalance of torque forces on 2,1 17,584 5/1938 Van Sittert 418/43 the rotor so as to cause rotation of the rotor in one direction or 2,719,512 10/1955 Kovach 418/267 the other with a minimum time lapse.

PATENTEDUEI 19 1911 3, 6 14,275 sum 1 OF 2 BY 41 dwa ATTORNEY CONTROL MEANS FOR BIDIRECTIONAL ROTARY FLUID MOTOR SYSTEM This invention relates to fluid power transmitting devices, and, more particularly, to a control means for a bidirectional, rotary fluid motor.

BACKGROUND OF THE INVENTION In fluid-actuating systems which comprise rotary fluid motors, such as the sliding vane type as exemplified in U.S. Pat. No. 2,719,512, screw-and-gear type or the like, employed to effect the operation or actuation of other devices, such as wing flaps or other control surfaces of an aircraft, a time lag exists between the input signal and the response of the system to that input signal. At least part of this reaction time is attributable to the interstices between components and other spaces in the fluid motors which provide leakage flow paths and fluid voids which must be filled with pressurized fluid before the fluid begins to become effective to operate the fluid motors by overcoming the load thereon. This delayed reaction time in some applications, such as in actuating systems for aircraft, especially ultrahigh-speed aircraft, is obviously undesirable and increases the hazards of flying.

ln the US. Pat. No. 3,016,021, to Rineer, the patentee recognized the desirability of flooding" a fluid motor to permit it to react quickly to input signal, and achieved this result with a unique, relatively complex, fluid motor in which the roller-type vanes functioned as valve elements to effect rotation in different directions in response to a control valve disposed in the fluid exhaust of return conduits from the fluid motor.

Accordingly, it is an object of this invention to provide a control means for a rotary fluid motor which permits the flooding of a bidirectional rotary fluid motor of conventional, relatively simple construction.

It is another object of the present invention to provide a control means in combination with a rotary fluid motor of conventional construction which enables the motor to switch from one direction of rotation to another quickly in response to an input signal.

A feature of this invention is the interconnection of a conventional bidirectional rotary fluid motor with a control means, source of pressurized fluid and a fluid exhaust or return means so that fluid pressure constantly exerts substantially equal and opposite torque forces upon the rotor of the fluid motor in the neutral or null position of a valve means of the control means to thereby prevent rotation of the rotor and maintain the rotor under the influence of these balanced opposing torque forces by keeping the fluid motor fully pressurized or in a flooded" condition. This feature provides for holding the fluid motor against rotation and provides rotation V of the fluid motor, when its operation is desired, within minimal reaction time.

SUMMARY OF THE INVENTION Thus, the present invention contemplates a power-transmitting device which comprises in combination, a novel control means and a conventional bidirectional, rotary fluid motor having rotor means connected to drive an output shaft in one direction or the opposite direction and plurality of fluid ports for the passage of pressurized fluid into and from the fluid motor. The control means, including the control valve means, is provided for controlling the flow of pressurized fluid to and from the fluid ports and thereby the direction and speed of rotation of the rotor means of the fluid motor. A pres surized fluid supply passageway means is connected to continuously communicate a source of pressurized fluid with the motor means through at least one of said fluid ports and the control valve means. An exhaust fluid passageway means is provided to communicate with the fluid motor means, through another of the multiplicity of fluid ports, to exhaust pressurized fluid from the fluid motor means and communicate with the control valve means. A fluid passageway means is provided to communicate with the fluid motor means, through two fluid ports other than the fluid ports to which the fluid supply and the exhaust passageway means communicate, and with the valve control means. The fluid control valve means has a moveable valve element constructed and arranged to control the intercommunication of the fluid supply passageway means, exhaust passageway means and the control passageway means and provide, in the neutral or null position, for the interruption of communication between the aforesaid three fluid passageway means.

In this neutral position of the control valve means, a fluid pressure in the control passageway means is provided which is the average of the fluid pressure in the supply and exhaust passageway means so as to exert a total torque upon the rotor means counter to and substantially equal in magnitude to the torque exerted by the fluid pressure conducted to the rotor means by the supply passageway means. With the fluid pres sure forces acting upon the rotor means in balance, the fluid motor is nonoperative. The movement of the valve element of the control valve means from the neutral or null position to communicate the control passageway means with the supply passageway means and thereby conduct supply fluid to the fluid motor, provides for the exertion of a total torque force upon the rotor means counter to and substantially double the torque force exerted by the supply fluid conducted to the fluid motor means by the supply passageway means. This imbalance of pressurized fluid acting upon the rotor means causes the latter to rotate, assuming such supply fluid pressure is sufficient to overcome the load upon the fluid motor means. The movement of the valve element of the valve control means to another position, whereby communication is effected between the fluid control passageway means and the exhaust passageway means so that the fluid pressure in the control passageway means is reduced to exhaust fluid pressure, an unbalanced torque force results from the fluid delivered to the rotor means by the supply passageway means so that rotation of the rotor means is achieved in a direction opposite to the direction of rotation effected in the first-mentioned nonneutral position of the control valve means.

The control valve means may be constructed and arranged to modulate fluid flow to the fluid motor means to control the speed of the fluid motor in either direction of rotation.

Since pressurized supply fluid is continuously delivered to the motor means, through the supply passageway means, and from the fluid motor means, through the exhaust passageway means in the neutral or null position of the control valve means, the fluid motor is maintained in a flooded" condition, thereby enabling the fluid motor to rapidly respond to the actuation of said valve control means.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be more fully understood from the following description thereof when considered in connection with the accompanying drawing wherein two embodiments of the invention are illustrated by way of example and in which:

FIG. l is a diagrammatic view of one embodiment of the present invention; and

FlGS. 2 and 3 are diagrammatic illustrations, similar to F 16. ll, showing other operative positions ofthe control valve;

FIG. 4 is a diagrammatic drawing of another embodiment of this invention; and

FIGS. 5 and 6 are views, similar to FIG. d, showing additional operative positions of the control valve shown in H6. d.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Now referring to the drawings and more particularly to FIGS. ll, 2 and 3, inclusive, in which one embodiment of the fluid-power-transmitting device according to this invention is diagrammatically illustrated, the reference numeral ll) generally refers to the fluid-power-transmitting device which comprises a fluid motor Ill and a control means of which control valve 12 fonns a part.

The fluid motor I l is of the conventional bidirectional sliding vane type which comprises a rotor 13, carrying slidable vanes 14, joumaled for rotation within a cavity of a housing so that the distal end edges of the vanes engage the inner surface of the housing cavity, the adjacent vanes and housing cavity defining working chambers into which pressurized fluid is introduced and exhausted to apply a torque force to the rotor. The fluid motor 11 may be of the type, such as exemplified in the US. Pat. No. 2,7l9,5 12, to Kovach, wherein the cavity of housing 15 is provided with two recesses, A and B, and four ports, 16, 17, 18 and 19, arranged so that ports 16 and 17 communicate with recess 19 while ports 18 and 19 communicate with recess B. With this construction, the fluid motor is provided with two sets of working chambers which by control of pressurized fluid flow into and from the ports 16, l7, l8 and 19, will achieve rotation of rotor 12 in a clockwise or counterclockwise direction. In accordance with the present invention, control means, including control valve 12, is provided to control direction and speed of operation of fluid motor 11.

The control valve 12 may be a three -way valve of any suitable construction, such as a spool valve, sleeve valve or the like. For illustration purposes only, control valve 12 is schematically shown in the drawings as a spool valve comprising a cylindrical housing 20 and an axially moveable valve element 21. The valve element has two spaced land portions 22 and 23 which define therebetween an annular groove 24. The housing 20 is provided wit three spaced ports 25, 26 and 27 which communicate with the interior of the housing. As will be readily apparent hereinafter, valve element 21 and the ports 25, 26 and 27 are so arranged and dimensioned that axial movement of valve element 21 will control communication between ports 25, 26 and 27. In addition to control valve 12, the control means includes pressurized fluid supply passage means or conduit 28, pressurized fluid exhaust passage means or conduit 28, pressurized fluid exhaust passage means or conduit 29, and a control passage means or conduit 30.

The pressurized fluid conduit 28 is in communication with a source of pressurized fluid (not shown), such as a pump or an accumulator, and with ports of control valve 12 and port 16 of fluid motor 11. Fluid conduit 28 communicates with port 25 of control valve 12, through a branch conduit 9.

The pressurized fluid exhaust conduit 29 is in communication with port 18 of fluid motor 11 and port 27 of control valve 12, via branch conduit 32, and with atmosphere or the recirculation pump, receiver or other similar component if the fluid system is a closed-recirculation system.

The control conduit 30 is connected to be in communication with port 26 of control valve 12 and with ports 17 and 19, through bifurcated branch portions 33 and 34, respectively.

While control valve 12 is shown hereinafter and described as dimensioned so that in the neutral position, the land portions of the valve element 21 close ports 25 and 27, it is within the purview of the present invention to proportion valve element 21 so that in the neutral position balanced communication exists between ports 25 and 27. In other words, without departure from the scope and spirit of the present invention, land portions 22 and 23 of valve element 21 may be constructed so as not to cut off in the neutral position communication between ports 25 and 27, but provide balance fluid flow therebetween.

As best seen in FIG. 1 when control valve 12 is in the neutral or null position, land portions 22 and 23 close ports 25 and 27 so that no communication exists between ports 25, 26 and 27. In this position, pressurized fluid, such as liquid or gas, is conducted to port 16 of fluid motor 11 and thence into chamber A. This pressurized supply fluid flow, at a pressure of P, fills the interstices between the components of the fluid motor (leakage flow paths) and other spaces or voids and becomes flooded." The pressurized fluid also flows into control conduit 30, via ports 17 and 18, to create a back pressure at each of the ports 17 and 18 of approximately one-half of the fluid pressure P at port 16. Since the fluid motor, through port 18 is in constant communication, via exhaust conduit 29, with a point of low fluid pressure, a constant flow of pressurized fluid through the fluid motor is provided and air or other gaseous fluids are not trapped in the fluid motor. Thus, the fluid motor is maintained in a flooded condition. Since the fluid pressure P, at port 16, acts to urge rotor 13 in a clockwise direction and the fluid pressure, at ports 17 and 19, exert a torque force on rotor 13 which is substantially equal and counter to the torque force imposed on the rotor by the fluid pressure at port 16, the forces acting on the rotor are balanced and rotor 13 remains stationary.

Actuation of control valve 12 to the position shown in FIG. 2, by slidable movement of valve element 21 to the left from the neutral position illustrated in FIG. 1, causes rotor 13 of fluid motor 11 to rotate in a counterclockwise direction. As shown, land portion 22 opens or uncovers port 25 in control valve 12 to bring port 25 into communication with port 26, via groove 24 so that fluid, at supply pressure P, exerts in chambers A and B through ports 17 and 19 of fluid motor 11, a torque force on rotor 13 in a counterclockwise direction. Since supply pressure P is exerted on the rotor in both chambers A and B for a total pressure of 2P, the supply pressure P exerting a clockwise torque on rotor 11 is overcome and rotor 11 is rotated in a counterclockwise direction. The response of the rotor to this input signal is very rapid because the motor is in a saturated or flooded" condition and no reaction timeis consumed in filling the leakage flow paths or voids of the fluid motor before the force is effective to achieve rotation of rotor To quickly effect clockwise rotation of rotor I3 of fluid motor 11, control valve 12 is actuated to the right to the position shown in FIG. 3 wherein land portion 22 closes port 25 and land portion 23 uncovers or opens port 27. In this position, the fluid in control conduit 30 is brought into communication with exhaust conduit 29, via groove 24 and branch conduit 32. This communication and the consequent dumping" of fluid in control conduit 30 provides, through port 16, a supply fluid pressure P which exerts a torque urging rotor 13 in a clockwise direction.

The fluid pressure in control conduit 30 is now at exhaust pressure and provides only negligible resistance to the rotation of rotor 13 in a clockwise direction.

The speed of rotation of rotor 13 in both the clockwise and counterclockwise direction is obviously achieved by positioning valve element 21 in any position between the neutral position and the position where ports 25 and 27 are fully open. Obviously, the more land portions 22 and 23 of valve element 21 restrict the flow area of ports 25 and 27 the slower will be the rotation of rotor 13. Thus, actuation of valve element 21 controls both direction and speed of rotation of rotor 13 and, hence, the direction and speed of an output shaft 31 to which rotor 13 is drivably connected.

In FIGS. 4, 5 and 6, a fluid-power-transmitting device 35 according to another embodiment of this invention is shown and which differs from the fluid-power-transmitting device 10 herein described and illustrated in FIGS. 1, 2 and 3 in that two motors of a unidirectional type are employed to effect the rotation of an output shaft instead of a single bidirectional fluid motor 11. The parts of fluid-power-transmitting device 35, corresponding to like or similar parts of transmitting device 10, will be designated by the same reference numbers, but with the suffix A added thereto.

The power-transmitting device 35 comprises two fluid motors 36 and 37 of a unidirectional type, such as an internal gear motor in which two meshing gears 38 and 39 are supported in a housing 40 for rotation, gear 38 being connected to rotate an output shaft 31A. In accordance with this invention a control means, including a control valve 12A, is provided to control the direction and speed of rotation of output shaft 31A.

The control valve 12A may be of any suitable type for control of flow through three ports and, since it is shown for illustration purposes only as being of identical construction to control valve 12, it will not be described in detail. As in the device herein described and shown in FIGS. l to 3, in addition to control valve 112A, the control means includes a pressurized fluid supply passage means or conduit 28A, pressurized fluid exhaust passage means or conduit 29A, and a control passage means or conduit 30A.

The supply passage means 26A is connected to communicate with fluid motor 36, via port 16A, to a source (not shown) of pressurized fluid at a pressure 1?. Supply passage means is also in communication with control valve 112A, via port 25A and branch line or pipe 9A.

The exhaust passage means 29A is connected to fluid motor 37 and to atmosphere or recirculating means (not shown) to conduct fluid from motor 37 to the latter.

The control passage means 30A is connected to communicate with fluid motors 36 and 37, via ports 17A and 19A, respectively, and with control valve 112A through port 26A.

in the operation of the power-transmitting device 35 with the control valve 112A in the neutral position shown in FIG. 4, output shaft 21A is stationary. As in power-transmitting device 10, fluid supply passage means 28A is constantly supplying fluid at a pressure P to fluid motor 36 to flood the motor and control passage means 30A. Since control valve 12A is positioned to close ports 25A and 27A, flow of fluid, through control passage means 30A, is closed off, except through fluid motor 37, via port 19A. With passage means 30A in communication with exhaust passage means 29A, through fluid motor 37, the latter is also maintained in a flooded condition. The fluid in control passage means 30A, through port 119A of fluid motor 37, exerts a torque force, through gears 36 and 39, on output shaft 311A in a direction counter and substantially equal to the torque force exerted on the output shaft by the fluid entering fluid motor 36 via port 16A. With the torque forces acting on output shaft 31A balanced, the output shaft is stationary.

To effect rotation of output shaft 31A in a clockwise direction, valve element 21A is actuated to the left from the position shown in FIG. 4 to the position illustrated in FIG. 5. The land portion 22A clears port 27A while land portion 23A maintains port 25A closed. In this position, control passage means 30A is brought into communication with exhaust passage means, via ports 26A and 27A, so that fluid in control passage means is released or dumped" to the exhaust passage means. When the fluid pressure in control passage means falls below supply pressure P and no longer exerts a torque force, through gears 38 and 39, on output shaft 31A in fluid motor 37 counter to that of the fluid in fluid motor 36, output shaft 31A is rotated in a clockwise direction. Any fluid flow through fluid motor 37 from control passage means to exhaust passage means 29A, by way of ports 119A and 13A, exerts only a negligible torque force counter to the rotation of output shaft 31A.

To reverse the rotation of output shaft 311A, valve element 21A of control valve 12A is moved to the right to the position shown in FIG. 6. In this position of valve element 21A land portions 22A and 23A close port 27A and open port 33A, respectively, so that supply passage means 23A is brought into communication with control passage means 36A. With the control passage means in communication with supply passage means, the fluid pressure in control passage means 30A rises to substantially a fluid pressure of P. Since the fluid entering fluid motor 36, via port 17A, is at a pressure substantially equal to the supply pressure P of the fluid entering fluid motor 36, via port 116A, the torque forces acting upon output shaft 31A in fluid motor 36 are substantially balanced. With control passage means communicating with exhaust passage means 29, through fluid motor 37, the fluid at supply pressure P flows into the fluid motor, via port 19A, to exert a counterclockwise torque force, through gears 38 and 39, on output shaft 31A. Since the torque forces are balanced in fluid motor 36, output shaft 31 will be rotated by fluid motor 37 in a counterclockwise direction.

To regulate the speed on the output shaft MA in either direction of rotation, valve element MA is positioned to control the flow areas of ports 27A and 23A between the fully opened and closed positions shown in FIGS. 5 and 6.

it is believed now readily apparent that the present invention provides a control means for a power-transmitting device which pennits the use of a conventional fluid motor means to achieve bidirectional rotation and rapid response to an input signal to change the direction of rotation.

Although two embodiments of the invention have been illustrated and described in detail, it is to be expressly understood that the invention is not limited thereto. Various changes can be made in the arrangement of parts without departing from the spirit and scope of the invention as the same will now be understood by those skilled in the art.

What is claimed is:

11. A hydraulic power-transmitting device comprising, in combination, a bidirectional rotary liquid motor means having housing means, rotor means defining with the housing means two sets of working chambers, and a first suction and discharge port means for one set of working chambers and second suction and discharge port means for the other set of working chambers to deliver pressurized liquid into the set of working chambers and exert a torque force on the rotor means and eflect thereby rotation of the rotor means in one direction or the opposite direction depending on which of said first and second suction and discharge port means pass liquid into and from the sets of working chambers, and control means com prising:

a. control valve means having first, second, and third fluid ports and movable means coacting with said ports to control flow of liquid therethrough;

b. uncontrolled pressurized liquid supply passage means communicating with a source of pressurized liquid and with the suction port means of said first suction and discharge port means of the fluid motor means and with said first fluid port of the control valve means;

c. uncontrolled exhaust passage means communicating with said discharge port means of said second suction and discharge port means of the liquid motor means and with the third fluid port to pass spent pressurized liquid from the working chambers of the liquid motor means;

d. control passage means communicating with the second fluid port of the control valve means, the suction port means of said second suction and discharge port means, and the discharge port of the first suction and discharge port means of the liquid motor means; and

c. said movable means of said control valve means coacting with said first, second and third fluid port to communicate, in one operative position, :said first and second fluid port and thereby affect rotation of the rotor means in one direction, in another operative position terminate communication between said first and second fluid ports and communicate the second fluid port with said third fluid port to affect rotation of the rotor means in an opposite direction, and in a neutral operative position of the movable means close off pressurized liquid flow through the first, second and third ports so that the communication between the pressurized liquid supply passage and the exhaust passage means is only through said suction port means of said first suction and discharge port means and the discharge port means of the second suction and discharge port means thereby maintaining said liquid motor means in a flooded condition and providing substantially balanced torque forces acting on the rotor means to prevent rotation of the latter.

2. The combination of claim 1 wherein said movable means comprises a slidable valve element movable relative to said first, second and third fluid ports communicating with the supply passage means, control passage means and exhaust passage means to vary the fluid flow areas of the said first, second and third ports.

3. The combination of claim 11 wherein said fluid motor means is of the balanced pressure slidable vane type.

and carrying radially extending, circumferentially spaced vanes slidable radially relative to the rotor as the latter rotates within the housing.

7. The combination of claim 1 wherein said control valve means comprises a spool having lands and a groove disposed for reciprocable movement relative to said fluid ports for controlling fluid flow through said fluid ports.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent 3,614,275 Dated October 19, 1971 Inventor(s) Richard S. Eibsen It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

IN THE SPECIFICATION:

Column 3, line 28, the word "wit" should read --with-- line 36 (actual line count) the numeral "28" should read --29 and the remainder of the line should be deleted.

Signed and sealed this 11th day of April 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents ORM PO-1050 [IO-69) USCOMM-DC BO37B-PO9 U 5 GOVERNMENT PRINTING OFFICE III! O-JGG-SS 

1. A hydraulic power-transmitting device comprising, in combination, a bidirectional rotary liquid motor means having housing means, rotor means defining with the housing means two sets of working chambers, and a first suction and discharge port means for one set of working chambers and second suction and discharge port means for the other set of working chambers to deliver pressurized liquid into the set of working chambers and exert a torque force on the rotor means and effect thereby rotation of the rotor means in one direction or the opposite direction depending on which of said first and second suction and discharge port means pass liquid into and from the sets of working chambers, and control means compRising: a. control valve means having first, second, and third fluid ports and movable means coacting with said ports to control flow of liquid therethrough; b. uncontrolled pressurized liquid supply passage means communicating with a source of pressurized liquid and with the suction port means of said first suction and discharge port means of the fluid motor means and with said first fluid port of the control valve means; c. uncontrolled exhaust passage means communicating with said discharge port means of said second suction and discharge port means of the liquid motor means and with the third fluid port to pass spent pressurized liquid from the working chambers of the liquid motor means; d. control passage means communicating with the second fluid port of the control valve means, the suction port means of said second suction and discharge port means, and the discharge port of the first suction and discharge port means of the liquid motor means; and e. said movable means of said control valve means coacting with said first, second and third fluid port to communicate, in one operative position, said first and second fluid port and thereby affect rotation of the rotor means in one direction, in another operative position terminate communication between said first and second fluid ports and communicate the second fluid port with said third fluid port to affect rotation of the rotor means in an opposite direction, and in a neutral operative position of the movable means close off pressurized liquid flow through the first, second and third ports so that the communication between the pressurized liquid supply passage and the exhaust passage means is only through said suction port means of said first suction and discharge port means and the discharge port means of the second suction and discharge port means thereby maintaining said liquid motor means in a flooded condition and providing substantially balanced torque forces acting on the rotor means to prevent rotation of the latter.
 2. The combination of claim 1 wherein said movable means comprises a slidable valve element movable relative to said first, second and third fluid ports communicating with the supply passage means, control passage means and exhaust passage means to vary the fluid flow areas of the said first, second and third ports.
 3. The combination of claim 1 wherein said fluid motor means is of the balanced pressure slidable vane type.
 4. The combination of claim 1 wherein said fluid motor means comprises two unidirectional fluid motors disposed adjacent to each other and connected to drive an output shaft in opposite directions.
 5. The combination of claim 4 wherein each of said unidirectional fluid motors is of the gear type.
 6. The combination of claim 1 wherein said fluid motor means comprises a housing, a rotor disposed in the housing and carrying radially extending, circumferentially spaced vanes slidable radially relative to the rotor as the latter rotates within the housing.
 7. The combination of claim 1 wherein said control valve means comprises a spool having lands and a groove disposed for reciprocable movement relative to said fluid ports for controlling fluid flow through said fluid ports. 